There is known a method of flash-butt resistance welding (S.I. Kuchuk-Yatsenko et al: "Kontaktnaya stykovaya svarka neprerivnym oplavleniem" 1976, Naukova dumka, Kiev, pp 59-62) in which the value of welding current is monitored and the speed of drawing together parts to be welded is changed depending on said value. With such a method, welded parts are not shorted for a long time.
There is also known a method of flash-butt resistance welding (SU, A, 1,065,121), wherein resistance between parts being welded is measured in the course of flashing, the current ratio of the resistance between parts being welded to short-circuit impedance of a welding circuit is monitored and, depending on the value of said ratio, the welded parts are brought together or moved apart.
The foregoing method allows increasing efficiency of a welding process due to a shorter time required to heat end faces of parts to a sufficient level. The above advantage is attributable to the fact that, in the known method, interruption of welding current flow and occurrence of shorts in flashing are made less probable by maintaining resistance between welded parts within predetermined limits.
However, in the afore-mentioned prior-art method no account is taken of a change in short-circuit impedance of a welding circuit in welding a batch of parts. In the course of utilization of a welding circuit its short-circuit impedance varies. A difference between the actual value of short-circuit impedance of a welding circuit and its rating may be as great as 20 to 30.10.sup.-6 .OMEGA.. Variations of said value may be influenced by a number of factors.
Firstly, gripping jaws and current buses of a welding circuit are heated in the course of welding, a feature substantially increasing short-circuit impedance of a welding circuit.
Secondly, as the welding operation is conducted, metal splashes stick to various parts of a welding circuit in an area encompassed by the welding circuit and to the components of said circuit, due to which its short-circuit impedance is also increased.
Thirdly, the magnitude of short-circuit impedance of a welding circuit is influenced by variations of contact resistance of current-carrying parts of said welding circuit.
Thus, the numerical value of short-circuit impedance of a welding circuit, which is used in process control, should be monitored and accounted for so that the flashing process is highly effective. In the foregoing method, no account is taken of this variation and the magnitude used is either the rating or the value of short-circuit impedance of a welding circuit obtained in maintenance operations. As a result, flashing stability is impaired and effectiveness of heating of parts is appreciably reduced. The welding time is increased, accordingly.